Non-volatile memories are memories where the information is conserved even if the memory is disconnected from a power source. In resistance change memories, the information (logic “1” and logic “0”) is stored as a “high” or a “low” resistance state of the memory cell. What non-volatile random access memories (NVRAM) have in common is that the information is programmed or erased in the power-on state, while the memory cell stores the information over a period of at least 10 years in the power-off state. Non-volatile resistance change memories address some of the issues of the ultra high density non-volatile memory market.
One type of non-volatile memory is FLASH memory. Prominent examples for FLASH applications are non-volatile memories in computers, cell phones, memory sticks, personal digital assistants (PDAs), digital cameras, smart cards, etc. Driven by the need to minimize production costs, the size of the memory cell has been continuously reduced. State of the art semiconductor devices are manufactured using the 65 nm technology (i.e., 65 nm denotes the size of the smallest feature of the manufactured structure). As a consequence of the continuous reduction of the memory cell size, the FLASH memory is expected to reach its physical scaling limit between the 45 nm and the 32 nm technology node.
A variety of resistance change memory concepts have been proposed to supersede FLASH. Resistance change memories can be categorized in phase change RAMs (PCRAM), resistive RAMs (RRAM) and electrochemical RAMs (ECRAM). Even though RRAM and ECRAM show some potential to become a successor of FLASH, the formation of a filament in the memory cell is essential for the memory operation. However, filamentary based memories do not scale well in relation to area or thickness. It remains therefore questionable, whether filamentary based memories can be commercialized. A disadvantage of PCRAM is the large current consumption of the memory during program/erase operations.
Some scalable non-volatile memories utilize a mixed electronic ionic conductor to store the information. Scalability of the memory cell is highly desired for memory applications. Mixed ionic electronic conductors are solids with a high mobility of ions or ion vacancies. The high mobility of ions/vacancies gives rise to ionic motion. At the same time, a sufficiently high concentration of electrons or defect electrons causes an electronic conduction. One of the properties of a mixed electronic ionic conductor is that a change in the ionic/vacancy concentration is correlated with a change in the concentration of electrons or electron holes. Thus, local changes in the electronic concentration affect the overall conductivity of the material. The mixed ionic electronic conductor will further be referred to as “solid state electrolyte”. Since the memory mechanism is very similar for solid state electrolytes with ionic and ion vacancies conduction, the focus is on solid state electrolytes with vacancy conduction. Similar considerations can be applied to solid state electrolytes of predominantly ion conduction.
By applying an external electric field, vacancies are redistributed inside the solid state electrolyte. Thus, the memory effect is based on the local change of electronic charge carriers, which is a consequence of the redistribution of vacancies. A concentration change of electronic charge carriers causes an increase or decrease in the resistance of the memory cell.
To ensure fast program/erase times, solid state electrolytes with high ionic mobility have to be used. During the programming operation, a concentration gradient of ions builds up. After switching off the external field, the redistributed ions tend to move back to their initial (equilibrium) positions, thereby lowering the concentration gradient. A redistribution of ions, however, causes a loss of the stored information over time, which can lead to a loss of the data stored by the memory cell. As a consequence thereof, fast electrochemical memories are often unreliable.
The above issues as well as others have presented challenges to the manufacture and implementation of electrochemical memories for a variety of applications.